Referring to FIG. 1, a conventional child safety seat includes a seat base 8 and a seat shell 9. The seat base 8 is installed on and secured to a vehicle seat 7 through an ISOFIX seat belt device. The seat shell 9 is mounted on the base seat 8 through a releasably latching mechanism so as to be removable from the seat base 8 to be alternatively installed on a stroller frame for facilitating carrying of a child. Referring to FIG. 2, the seat shell 9 includes a bottom portion 91 disposed to be connected with the seat base 8 or a stroller frame, a backrest portion 92 pivotably mounted to a rear end of the bottom portion 91, and a footrest portion 93 pivotably mounted to a front end of the bottom portion 91. Hence, the backrest portion 92 and the footrest portion 93 can be tilted relative to the bottom portion 91, so that the tilt angle of the backrest portion 92 and the footrest portion 93 can be adjusted appropriately for providing a satisfied feeling of comfort to a child.
Referring to FIG. 3, a safety angle (a), which is included by the backrest portion 92 and an upright direction, must not be more than 70 degrees as required by the safety rules of a crash test. However, since the seat base 8 generally has a horizontal rested surface for the seat shell 9 to rest thereon, the backrest portion 92 may be tilted to have the included angle be more than 70 degrees when a whole flat seating surface of the seat shell 9 is desired, and thus over-reclining of the safety seat might occur.